1. Field of the Invention
The present invention relates to air-fuel ratio detection using a current limited (limit-current) type air-fuel ratio sensor which outputs a current signal corresponding to an air-fuel ratio in a detection object gas upon application of a voltage to the sensor.
2. Related Art
In recent years, in air-fuel ratio control for vehicle engines, there are needs for increased control precision and lean burning. To respond to these needs, a linear air-fuel ratio sensor which widely and linearly detects the air-fuel ratio of the gaseous mixture (oxygen concentration in an exhaust gas) supplied into an engine, and an air-fuel ratio detecting apparatus using this sensor are employed. As such air-fuel ratio sensor, in a limit-current air-fuel ratio sensor, its limit-current detection region shifts in accordance with an air-fuel ratio (oxygen concentration), as it is well known. More specifically, in the known V-I characteristic graph, the limit-current detection region comprises a linear segment parallel to the axis V, and the region shifts to the positive voltage side as the air-fuel ratio moves toward the lean side, while it shifts to the negative voltage side as the air-fuel ratio moves toward the rich side. Accordingly, if a voltage applied to the sensor has a fixed value when the air-fuel ratio changes, it is impossible to perform precise air-fuel ratio detection using the limit-current detection region (the linear segment parallel to the axis V) in the entire air-fuel ratio detection range.
Accordingly, a technique to variably set a voltage applied to the air-fuel ratio sensor in accordance with the air-fuel ratio (sensor current) is desired. As this type of prior art, Japanese Patent Publication No. 7-18837 discloses "air-fuel ratio detecting apparatus" which instantly cuts off the voltage applied to the air-fuel ratio sensor to detect sensing element internal-resistance from an electromotive force and current at that time, and variably sets the applied-voltage based on the sensing element internal-resistance. Further, the "air-fuel ratio sensor" disclosed in Japanese Patent No. 2509905 uses a technique to stepwise change an applied-voltage in accordance with whether the air-fuel ratio is on the rich side or the lean side.
However, since a zirconia sensor, which is generally used as a linear air-fuel ratio sensor, has a capacitive (condenser) characteristic, the above conventional techniques cause the following problem. FIG. 27 shows an equivalent circuit of the air-fuel ratio sensor. In the equivalent circuit, reference symbol Rg denotes a particle resistance of solid electrolyte (zirconia sensing element) with respect to oxygen ions; Rh and Ch, respectively a boundary resistance and a boundary capacitance of the surface of the particle of the solid electrolyte; Rf and Cf, respectively an electrode surface resistance and an electrode surface capacitance. In this case, as shown in FIG. 28, when the voltage applied to the air-fuel ratio sensor is changed, the current of the sensor causes a peak current immediately after voltage change due to the influence of electric charge on the above values Ch, Cf, which, as a result, prolongs the time required until the current converges into a predetermined current value.
This situation is especially regarded as a serious problem for realizing high-precision air-fuel ratio feedback control, however, the techniques disclosed in the above publications have not solved this problem. As a result, in a region, where an air-fuel ratio should be detected with high precision, such as a stoichiometric control region (a region having air-fuel ratio=about 13 to 17), air-fuel ratio detection precision is degraded, and further, air-fuel ratio control precision might be lowered to cause inconveniences such as degradation of emission.